This invention relates generally to transistor amplifiers and more particularly to transistor amplifiers having reduced parasitic, or parametric, oscillations.
As is known in the art, transistor amplifiers, particularly high power pHEMT and HBT power amplifiers, frequently have oscillations that occur only when the amplifier is driven by a large signal. Under small signal conditions these oscillations are usually not present. Such oscillations are sometimes referred to as parametric oscillations since they depend on variations of particular external parameters (bias, frequency, input drive, and temperature). While the amplifier may appear perfectly stable under small signal conditions, oscillations can appear as the amplifier is driven harder. These oscillations tend to be very sensitive to input drive, bias conditions, and operating frequency. FIG. 1 shows a power amplifier 10. The amplifier includes a plurality of transistor devices 12 arranged as shown. Several typical observated oscillations in such type of amplifier 10 are shown in FIGS. 4 through 7. FIG. 4 shows subharmonic f/2 and 3f/2 oscillations. FIG. 5 shows a 200 MHz spurious oscillation that appeared as an amplifier was driven about 0.5 dB into compression. FIGS. 6 and 7 show further examples of troublesome spurious oscillations under power drive. Additional examples may be found in "Power Amplifiers: From Milliwatts to Kilowatts . . . Cool Devices with Hot Performance, " Short Course Notes of Aryeh Platzker's section, 1998 GaAs IC Symposium. Existence of such oscillations can be a major problem in many wide band radar applications, where the oscillation tones could be mistaken for false signals. Elimination of these oscillations is essential for the system to work properly.
One technique commonly used to reduce these parasitic oscillations is to use a parallel R-C filter as presented in "Power Amplifiers: From Milliwatts to Kilowatts . . . Cool Devices with Hot Performance," Short Course Notes Steve Nelson and Aryeh Platzker's section, 1998 GaAs IC Symposium. Thus, referring to FIGS. 1 and 2, the transistor devices 12 in FIG. 1 are replaced with the transistor devices, such as transistor device 12' shown in FIG. 2. Thus, here each device includes a plurality of transistor cells 15, here each a FET, having the gate electrodes G connected to a common node 16. A filter 18, i.e., the R-C network, is connected between an input node 20 and the common node 16. These R-C filters (i.e., networks) 18 are generally placed on the gate manifold (i.e., the common node 16) of the transistor cell 15, as shown in FIG. 2, is somewhat removed from the intrinsic transistor device.
As is also known in the art, drive dependent oscillations were first studied in the 1920s and 30s for tube amplifiers. Van Der Pol first studied how nonlinear resistance can introduce forced oscillations. Reference is made to "Forced Oscillations in a Circuit with Non-Linear Resistance (Reception with reactive Triode)", by Balth Van Der Pol, published by The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science, series 7, vol. III, no. Jan. 13, 1927, pp. 65-81 and to Nonlinear Oscillations, by Nicholas Minorsky, published by D. Van Nostrand Company, Princeton, N.J. 1962, page 241. Mandlestam and Papalexi further investigated subharmonic oscillations in electron tubes, as reported in Nonlinear Oscillations, by Nicholas Minorsky, published by D. Van Nostrand Company, Princeton, N.J. 1962, page 469.
Over the years, numerous authors have investigated the subject of large signal oscillations. See: Otward Muller and William Figel, "Stability Problems in Transistor Power Amplifiers," Proceedings of the IEEE, Aug. 1967, pp. 1458-1466; W. Mumford, "Some Notes on the History of Parametric Oscillations," Proceedings of the IRE, May 1960, pp 848-850; R. Phillips, "Parametric Oscillation in a Damped Resonant System," IEEE Transactions on Circuit Theory, December 1963, pp. 512-515; J. Manley and H. Rowe, "Some General Properties of Nonlinear Elements--Part 1. General Energy Relations," Proceedings of the IEEE, July 1956, pp. 904-913.
Subharmonic oscillations (f/2, 3f/2, etc.) have been investigated in depth and many parallels to pumped varactor diodes can be made to explain the phenomenon in pHEMT devices. It is well known that a pumped varactor diode gives rise to subharmonic components due to the nonlinear capacitance. See also P. Penfield, Jr. & R. Rafuse, Varactor Applications, The MIT Press, Cambridge, Mass, 1962. To first order, the gate of a FET can be analyzed as a pumped varactor diode; a primary contributor to subharmonic oscillations being the nonlinearity in Cgs and Cgd, where Cgs is the gate to source capacitance and Cgd is the gate to drain capacitance. See also J. Imbornone, M. Murphey, R. Donahue, E. Heaney, "New Insight into Subharmonic Oscillation Mode of GaAs Power Amplifiers Under Severe Output Mismatch Condition," 1996 GaAs IC Symposium, pp. 307-310. A simplified FET model is shown in FIG. 8 and its equivalent input impedance is shown in FIG. 9, where:
G is the gate; PA1 D is the drain; PA1 S is the source; PA1 IDS is the drain to source current; PA1 Rg is the gate resistance; PA1 Cgs is the gate to source capacitance; PA1 Cdg is the drain to gate capacitance; and PA1 Cds is the drain to source capacitance.
Assuming the nonlinear capacitance, C(t), shown in FIG. 3, varies as: EQU C(t)=C.sub.o +C.sub.2 sin(2.omega..sub.o t)
where t is time and the pumping frequency (input drive frequency) is 2.omega..sub.o, oscillations will arise at half the drive frequency provided ##EQU1##
where 2.omega..sup.o is the input signal frequency, .omega..sub.o is the frequency of the f/2 parasitic oscillation, Rs is the series resistance of the diode (analogous to the gate resistance, Rg), Zl is the load impedance seen by the diode at .omega..sub.o (analogous to the impedance looking back from the gate of the FET), C.sub.o is the small signal capacitance, and C2 is the nonlinear component of the varactor capacitance. Despite the oversimplification of this theory, it qualitatively describes the behavior of pHEMT power amplifiers. Subharmonic oscillations will not appear under low drive (C2.fwdarw.0), or if Rs is high enough. Adding the R-C filter 18 (FIG. 2) increases the input resistance enough at low frequencies to eliminate oscillations under drive.
The subject of spurious parasitic oscillations has been studied less, but is also due to nonlinearities in the transistor. Some insight into spurious parasitic oscillations may be found in the above-mentioned reference by Otward Muller and William Figel. There the authors view the amplifier as the superposition of a nonlinear amplifier and a linear amplifier with the bias point determined by input drive. The nonlinear amplifier causes subharmonic oscillations while linear amplifier component causes spurious oscillations. Driving the device hard results in a negative input impedance which in turn can cause oscillations. The resistance in the R-C filter 18 (FIG. 2) offsets the negative resistance in the device, thereby suppressing any oscillation.